1. Field of the Invention
The present invention generally relates to a mobile communications system using a spread spectrum (SS) scheme, and in particular to transmitting and receiving circuits for use in the mobile communications system.
2. Description of the Related Art
There have been known the following schemes for cellular mobile terminals and base stations: Personal Digital Cellular (PDC: RCR STD 27) of the RCR (Research & Development Center for Radio System) in Japan, Interim Standard 54 (IS 54) of the TIA (Telecommunications Industry Association) in North America, and Global System for Mobile Communications (GSM) of the ETSI (European Telecommunications Standard Institute) in Europe, are Time Division Multiple Access (TDMA) systems. Further, Interim Standard 95 (IS 95) released by TIA in North America is a Code Division Multiple Access (CDMA) system.
The RCR and IS 54 have adopted a .pi./4-shift DQPSK (Differential Quadrature Phase Shift Keying) modulation scheme, providing a reduced amplitude variation which permits a transmitter to employ a high-efficiency power amplifier. The .pi./4-shift DQPSK modulator and demodulator are described in an article written by Shimizu et al., "Design Concept of TDMA Cellular Mobile Radio Units" (IEEE VTC '92). The GSM has adopted another modulation scheme, Gaussian Filtered Minimum Shift Keying (GMSK). The GMSK modulation provides a smoothing of the phase transitions at symbol boundaries and preserves a constant-envelope property of a transmitted signal. Compared to the .pi./4-shift DQPSK modulation, however, the GMSK modulation causes the spectrum to be expanded, resulting in reduced bandwidth-efficiency.
On the other hand, is the IS 95, according to the document released by TIA (TIA/EIA/IS 95, pp 6-8), in a reverse channel or uplink, Binary PSK and Offset QPSK are used as the first modulation and the second modulation, respectively. In a forward channel or downlink, Binary PSK and QPSK are used as the first modulation and the second modulation, respectively.
In CDMA systems such as IS 95, the first modulation cannot adopt QPSK but only Binary PSK. In the uplink, only a combination of Binary PSK for the first modulation and Offset QPSK for the second modulation is permitted in a CDMA system. The reason is that the QPSK for the use of first modulation may cause undesired phase transitions of .pi. at symbol boundaries, resulting in the loss of the advantage that Offset QPSK provides a very small amplitude variation. See the crossed dashed lines in FIG. 10B.
Since the symbol rate of Binary PSK is twice as high as that of QPSK in the case of the same bit rate, a spreading ratio of Binary PSK, which is defined as the number of chips per symbol duration, is reduced to a half that of QPSK, causing a reduced length of spreading code. Especially when an adaptive despreading filter is used to eliminate interference on the same frequency, a reduction of the spreading code length becomes a significant problem because the number of interference waves to be eliminated depends on the spreading ratio. Therefore, if possible, the first modulation preferably should adopt QPSK rather than Binary PSK.
Further, in the case of Offset QPSK, it is necessary for a receiver to sample a received signal at a sampling rate of twice the chip rate to obtain a correlation value. In the case of QPSK or .pi./4-shift QPSK, the sampling rate is equal to the chip rate. Therefore, in a broadband CDMA system adopting the Offset QPSK modulation scheme, it is more difficult to implement its hardware as the chip rate increases.
Furthermore, in the case of Offset QPSK, the respective signal points of a received signal are smeared into a circular shape like a cloud on a four-phase signal constellation at sampling time points as shown in FIG. 10B. Therefore, a possibility of interference between in-phase and quadrature-phase components arises, resulting in deteriorated characteristics of the receiver.